1. Field of the Invention
The present invention relates to a method of manufacturing a semiconductor device. In particular, it relates to a method of forming a concave portion in an interlayer insulating film comprised of two films: a silicon nitride film or a silicon nitride oxide film at the bottom and a silicon oxide film at the top.
2. Description of Related Art
As semiconductor devices are highly integrated, elements of the semiconductor devices such as transistors are miniaturized. Further, it is getting common to use a layered substrate provided with plural interlayer insulating films and metal wiring layers inserted between interlayer insulating films as a semiconductor substrate for the semiconductor devices.
The layered substrate has contact holes which are opened on the metal wiring layer and reach to the semiconductor substrate through the interlayer insulating film for electrical connection between the metal wiring layer on the interlayer insulating film and the semiconductor substrate.
Further, in order to establish electric connection between upper and lower metal wiring layers, the layered substrate has via holes which are opened on the upper metal wiring layer and reach to the lower metal wiring layer through the interlayer insulating film.
The contact holes and via holes are usually provided by forming openings in the interlayer insulating film made of silicon oxide by etching.
For example, where the semiconductor devices have transistors formed on the semiconductor substrate and the transistors are designed in the order of ¼ micron or smaller, the contact hole is formed to connect to both a source/drain region and a device isolation region of the transistor.
In order to form such a contact hole by etching, attention must be paid so as not to etch the device isolation region made of silicon oxide film. For that purpose, a silicon nitride film of several hundred Å thick is formed as an etch stopper under the interlayer insulating film.
That is, the interlayer insulating film is formed of the silicon nitride film and the silicon oxide film stacked in layers.
Accordingly, to form the contact hole, an opening is formed in both of the silicon oxide film and the silicon nitride film by etching.
Specifically, the silicon oxide film is first etched with an etching gas capable of etching the silicon oxide film at an etching rate higher than that for the silicon nitride film. This etching is stopped at the surface of the silicon nitride film which serves as the etch stopper.
Then, the silicon nitride film is etched with an etching gas capable of etching the silicon nitride film at an etching rate higher than that for the silicon oxide film. Thus, the contact hole is formed.
On the surface of the metal wiring layer, a silicon nitride oxide film is formed as an anti-reflection film used for forming a resist pattern for patterning the metal wiring layer by photolithography.
Accordingly, the interlayer insulating film on the metal wiring layer is formed of the silicon nitride oxide film and the silicon oxide film stacked in layers.
That is, in order to form the via hole, an opening is formed in both of the silicon oxide film and the silicon nitride oxide film by etching.
Specifically, the silicon oxide film is first etched with an etching gas capable of etching the silicon oxide film at an etching rate higher than that for the silicon nitride oxide film, and then the silicon nitride oxide film is etched with an etching gas capable of etching the silicon nitride oxide film at an etching rate higher than that for the silicon oxide film. Thus, the via hole is formed.
In order to form the contact hole or via hole, it is necessary to etch the silicon oxide film formed on the silicon nitride film or silicon nitride oxide film with an etching gas showing higher selectivity to the silicon oxide film than to the silicon nitride film or the silicon nitride oxide film. In this case, generation of an excess of fluorine ions and fluoric radicals must be prevented, so that C4F8, C5F8 or the like having high C/F atomic ratio is used as the etching gas.
Where the silicon oxide film is etched using such an etching gas, however, a reaction product such as fluorocarbon is generated and deposited at the bottom of the hole when the silicon nitride film or silicon nitride oxide film is exposed.
In the case of forming the contact hole through the silicon nitride film or the silicon nitride oxide film by etching, an etching gas showing higher selectivity to the silicon nitride film or the silicon nitride oxide film than to the underlying silicon substrate or silicon oxide film of the device isolation region is used. Further, in the case of forming the via hole in the same manner, an etching gas showing higher selectivity than to the underlying metal wiring layer is used.
However, also in the case of etching the silicon nitride film or the silicon nitride oxide film with such an etching gas, a reaction product such as fluorocarbon is deposited at the bottom of the resulting hole.
If a metal plug of W or Cu is formed in the thus formed contact hole or via hole in which the fluorocarbon reaction product remains, contact resistance or via resistance increases, which results in reduction of LSI wiring reliability and yield of the devices.
To deal with such a problem, commonly known is a method of forming the metal plug after removing the fluorocarbon reaction product from the bottom of the resulting contact hole or via hole using an exfoliative washing solution.
Further, also known is a method comprising a pretreatment of removing the reaction product from the bottom of the hole using plasma containing argon, hydrogen and oxygen performed before sputtering a barrier metal in the process of forming the metal plug (see Japanese Unexamined Patent Publication No. HEI 10 (1998)-256232).
If the reaction product is removed with the washing solution, however, it will be difficult to wash away the washing solution as the aspect ratio of the contact hole or via hole increases. Accordingly, the metal plug may possibly be formed in the hole in which the washing solution remains.
Where the metal plug is formed in the hole in which the washing solution remains, the washing solution is thermally reacted with the underlying silicon or metal wiring material with the help of heat generated by the formation of a tungsten film as the metal plug, and converted to a non-conductive film.
Further, in the case where the reaction product is removed by the pretreatment using the plasma, the wafer is taken out of an etching system used for forming the contact hole or via hole, exposed to atmospheric air, and then introduced to a system for the pretreatment before the barrier metal sputtering.
When the wafer is exposed to the atmospheric air, the reaction product deposited at the bottom of the hole is reacted with moisture in the atmosphere and converted into a film which is difficult to remove. Accordingly, complete removal of the reaction product would be difficult.